Surface materials of workpieces may be adversely affected by corrosive environments in various different industries. In the petroleum industry, corrosion of metallic surfaces by petroleum materials occurs at different stages of production and distribution. To reduce this corrosion, inhibitors have been utilized as a corrosion control method. There are various inhibitors which work in different ways, as well as various screening procedures used for the selection of inhibitors. Some inhibitors work by neutralizing active ions, others by reducing ion mobility and others by changing the ion transport numbers. In all cases the electrical conductivity of the corrosive fluid is altered, and various electrical parameters contributing to the overall corrosion mechanism will be affected. Accordingly, by using the corrosive fluid as an electrolyte in which two electrodes are immersed, and by measuring electrical characteristics of the electrolytic circuit, an indication of the level of corrosion which continues to occur may be derived. U.S. Pat. No. 5,370,776 describes a technique that utilizes the “break point frequency” (frequency at 45° phase angle) in order to determine the effectiveness of a corrosion inhibitor in preventing corrosion of metallic surfaces using electrochemical impedance spectroscopy (EIS).
In the magnetic recording industry, one corrosion auditing technique uses a scanning electronic microscope (SEM) to visually evaluate overcoat materials. Such a visual technique is time consuming and can sometimes yield false positives.
Another corrosion evaluation technique, used in the magnetic recording industry, is described in U.S. Pat. No. 6,512,382. Magnetic heads coated with diamond-like carbon films are tested by exposing the heads to a salt environment similar to that found in a production process. By comparing the sensor resistance values measured immediately following salt application and after exposure under different conditions (varying temperature, humidity and time), the susceptibility of the magnetic head to corrosion is determined.
Yet another solution, though not specifically for magnetic heads, is described in U.S. Pat. No. 4,806,849. The degradation level of a sample is determined by first dividing the sample into several zones using a grid. Impedance spectra are obtained from each area, and the degree of degradation is ranked based on the statistical distribution of the measured impedance values.